1. Field of the Invention
The present invention relates to a laminated draw-formed container having excellent flavor-retentivity and shock resistance (dent resistance). More specifically, the invention relates to a container which is obtained by draw-forming a material comprising a metal plate on which is laminated a polyester film, the container exhibiting improved performance for retaining flavor of the content and improved dent resistance.
2. Description of the Prior Art
Conventional side seamless cans can be represented by those obtained by subjecting a metal blank such as an aluminum plate, a tin plate or a tin-free steel plate to at least one stage of draw working between a drawing die and a punch thereby to form a cup consisting of a barrel without side seam and a bottom that is connected to the barrel without forming seam, and then, as required, subjecting the barrel to the ironing between an ironing punch and a die to reduce the thickness of the barrel of the container. It has also been known to reduce the thickness of the side wall relying upon the bend-elongation by using the corner portion of a redrawing die (Japanese Patent Publication No. 501442/1981).
As a method of covering side seamless cans with an organic material, furthermore, there has been known a method of laminating a resin film on a metal blank of before it is formed into a can in addition to the generally employed method of applying an organic paint onto the formed cans. Japanese Patent Publication No. 34580/1984 teaches use of a metal blank on which is laminated a polyester film derived from terephthalic acid and tetramethylene glycol. It has further been known to produce redraw-formed cans relying upon the bend-elongation by using a metal plate coated with a vinyl organosol, an epoxy, a phenolics, a polyester or an acryl.
Japanese Patent Application No. 101930/1991 filed by the present inventors discloses a coated metal plate for cans comprising a laminate of a metal plate, a polyester film layer composed of an ethylene terephthalate unit as a chief component and, as required, an adhesive primer layer interposed between the metal plate and the polyester film layer, said polyester film layer having an X-ray diffraction intensity ratio as defined by the following formula (1), EQU R.sub.X =I.sub.A /I.sub.B ( 1)
wherein I.sub.A denotes an X-ray diffraction intensity by a diffraction plane which is in parallel with the surface of the polyester film and having a spacing of about 0.34 nm (CuK.alpha. X-ray diffraction angle of from 24.degree. to 28.degree.), and IB denotes an X-ray diffraction intensity by a diffraction plane which is in parallel with the surface of the polyester film and having a spacing of about 0.39 nm (CuK.alpha. X-ray diffraction angle of from 21.5.degree. to 24.degree.), PA1 wherein n.sub.1 is a refractive index in the direction of a maximum orientation of the film, and n.sub.2 is a refractive index in the direction of thickness of the film,
of from 0.5 to 15 and an aniostropic index of in-plane orientation of crystals of not greater than 30.
It has long been known to laminate polyesters in the form of a plurality of layers on a base member such as a metal. For instance, U.S. Pat. No. 2,961,365 discloses a laminate obtained by laminating a linear terephthalate polyester via an adhesive layer of a polyester which comprises a polymethylene glycol with 2 to 10 carbon atoms and isophthalic acid, hexahydroterephthalic acid, or a dibasic acid of a combination thereof with terephthalic acid.
Japanese Laid-Open Patent Publication No. 176144/1991 discloses a laminated material having excellent flavor retentivity comprising a co-extruded resin laminate of a first resin layer of amorphous or low-crystalline saturated polyester resin having a glass transition temperature of not lower than 40.degree. C. and a second resin layer of a crystalline polybutylene terephthalate resin which is a copolymer of a 1,4-butane diol and terephthalic acid.
Moreover, Japanese Laid-Open Patent Publication No. 42643/1993 discloses a biaxially oriented polyester film that is used being laminated on a metal, the surface layer of the polyester film of the side laminated on the metal having a melting point of not lower than 170.degree. C. but not higher than 220.degree. C. and the surface layer of the polyester film of the side not laminated on the metal having a melting point of not lower than 220.degree. C. but not higher than 255.degree. C.
With the metal blank coated with an organic film in advance, however, the organic film is subject to be damaged by tools in the draw-forming step, and the metal is actually or latently exposed at portions where the film is damaged, permitting the metal to be eluted out or corroded at such portions. In producing the seamless cans, furthermore, there takes place such a plastic flow that the size increases in the direction of height of the can and the size decreases in the circumferential direction of the can. This plastic flow causes the adhesion force to decrease between the surface of the metal and the organic film and further causes the adhesion force to decrease between them with the passage of time due to residual distortion of the organic film. Such a tendency becomes conspicuous particularly when the content is hot-canned or when the canned content is sterilized at low temperatures or at high temperatures.
In the case of a conventional draw-formed can made of a laminate of a polyester composed of a polybutylene terephthalate (PBT) as a chief component, adhesion property to the metal base plate is favorable and workability is fairy good when the crystallinity is suppressed low leaving, however, a problem in that barrier property of the film against the corrosive component is of the order of about one-half that of polyethylene terephthalate/isophthalate (PET/I) permitting the film to stick to the tools, as well as a problem in regard to heat resistance of the container.
In the case of a draw-formed can using a laminate of a film of polyethylene terephthalate (PET) or PET/I, on the other hand, excellent barrier property is obtained against corrosive components and fairly good workability is obtained accompanied, however, by a problem in that the coating film of the can loses shock resistance and, particularly, dent resistance to a great degree with the passage of time after the content is packaged in the obtained container.
In this specification, the dent resistance stands for a test of determining whether the film of the container can withstand the breakage or peeling when impact or shock is given to the container to such a degree as to leave a trace of hit, and serves as a very important item for evaluating the practical durability of the canned content.
A proposal for using a plurality of polyester layers is to eliminate the defect of the case of when a single polyester layer is used. With the combination of a plurality of polyester layers proposed so far, however, problems still remain in obtaining a deep-draw-formed container and in regard to shock resistance and, particularly, dent resistance of the draw-formed container with the passage of time. Besides, the containers are not still satisfactory in regard to flavor-retentivity of the content.
It has been known that the container having the internal surface that comes into contact with the content and is composed of a thermoplastic resin exhibits off-flavor (loss of flavor of the content) to a striking degree compared with the contains having the internal surfaces coated with an epoxy-phenol paint.
The off-flavor of the container using a thermoplastic resin inclusive of polyester as an inner surface material is attributed to, first, that the flavor components of the content is adsorbed by the resin film layer and, second, that low-molecular components in the resin film and metal components under the film elute out into the content.
Employment of a laminated layer structure of a polyester film on the surface of the metal and employment of a copolymerized polyester layer having a low crystallinity or a low melting point as the lower layer that comes into contact with the metal, are effective in increasing the adhesion between the metal and the film, in enhancing the draw workability and in improving the shock resistance or the dent resistance accompanied, however, by a decrease in the crystallinity, a drop in the melting point and, hence, permitting the flavor components to be adsorbed to a large extent. Moreover, the low-molecular-weight components increase and the tendency of elution increases, too.